1. Field of the Invention
The present invention relates to an image signal processing device for processing an image signal.
2. Description of the Related Art
In the field of image signal processing devices, an image signal processing device has heretofore been known which is arranged to form a carrier chrominance signal by performing quadrature two-phase modulation of two kinds of color-difference signals R-Y and B-Y.
In general, such an image signal processing device is arranged to form the carrier chrominance signal by performing quadrature two-phase modulation of two kinds of analog color-difference signals R-Y and B-Y by means of a balanced-modulation circuit or the like.
In one recent type of image signal processing device, two kinds of analog color-difference signals R-Y and B-Y are each digitized at a sampling frequency which is an integer multiple (for example, four times) of a color subcarrier frequency, and the obtained two kinds of digital color-difference signals R-Y and B-Y are arranged in alternate sequence, for example, as shown in FIG. 1. A carrier chrominance signal is formed from the two kinds of digital color-difference signals R-Y and B-Y by using a digital color encoder for forming a quadrature two-phase modulated carrier chrominance signal by inverting, as shown in FIG. 1, the polarities of the two kinds of digital color-difference signals R-Y and B-Y in a predetermined alternate manner according to whether the NTSC television system or the PAL television system is used as a television signal system.
However, the former arrangement for encoding two kinds of analog color-difference signals R-Y and B-Y into a carrier chrominance signal in the form of analog signals has a number of disadvantages when compared to the latter arrangement for processing two kinds of color-difference signals R-Y and B-Y in the form of digital signals. For example, a carrier leak or the degradation of a carrier balance may occur in an encoded carrier chrominance signal, no good stability is attained with respect to temperature variations, and complicated adjustment is needed.
The latter arrangement for encoding two kinds of analog color-difference signals R-Y and B-Y into a carrier chrominance signal by digital processing has the disadvantage that no satisfactory versatility is attained since the sampling frequency for use in digitizing the two kinds of analog color-difference signals R-Y and B-Y must be an integer multiple of the color subcarrier frequency. If the digital color-difference signals R-Y and B-Y are signals digitized at a sampling frequency other than the integer multiple of the color subcarrier frequency, it is necessary to carry out processing according to the method illustrated in FIG. 1 after the sampling frequency for the digital color-difference signals R-Y and B-Y has been subjected to rate conversion and re-sampling of the digital color-difference signals R-Y and B-Y has been performed at a frequency which is the multiple integer (for example, four times) of the color subcarrier frequency. However, complicated processing is needed in the rate conversion for converting color-difference signals digitized at an arbitrary sampling frequency other than the integer multiple of the color subcarrier frequency into digital signals corresponding to the sampling frequency which is the integer multiple of the color subcarrier frequency. To achieve such processing, a complicated arrangement is needed and an increase in cost is incurred.
The currently used television signal systems are divided into three major systems: the NTSC television system used in Japan, North America, etc., the PAL television system used in China, Western Europe, etc., and the SECAM television system used in Eastern Europe, etc.
Each of the NTSC and PAL systems adopts a format in which a chrominance signal formed by performing quadrature two-phase modulation of a color-subcarrier-frequency signal in accordance with two kinds of color-difference signals R-Y and B-Y is multiplexed with a luminance signal (Y signal). A color subcarrier frequency fsc of the color-subcarrier-frequency signal is 3.579545 MHz in the case of the NTSC television system or 4.43361875 MHz in the case of the PAL television system.
FIG. 2 is a schematic block diagram showing the construction of a conventional signal processing device for forming a composite video signal from a luminance signal and two kinds of color-difference signals.
Referring to FIG. 2, a Y signal is digitized by an A/D converter 1Y at a predetermined sampling frequency fs, and the digitized Y signal is stored in a frame memory (Mem) 2Y. In the meantime, color-difference signals R-Y and B-Y are respectively digitized by A/D converters 1R and 1B at an arbitrary frequency synchronized with the color subcarrier frequency fsc, and the digitized color-difference signals R-Y and B-Y are stored in frame memories 2R and 2B, respectively.
In the above-described signal processing device, the color-difference signals R-Y and B-Y are digitized in the respective A/D converters 1R and 1B at a frequency synchronized with the color subcarrier frequency fsc, for example, at a frequency 2 fsc, 4 fsc, fsc or fsc/2, while the Y signal can be digitized at an arbitrary frequency. However, to make it easy to constitute a not-shown controller for controlling the read/write operation of each of the frame memories 2Y, 2R and 2B, the Y signal is also digitized in the A/D converter 1Y at a frequency synchronized with the color subcarrier frequency fsc, for example, at a frequency 4 fsc or 3 fsc.
The digital Y signal stored in the frame memory 2Y is read out and, then, converted into an analog signal by a D/A converter 4Y which operates at a sampling frequency fs. The analog signal is applied to a low-pass filter (LPF) 5Y, and a signal passed through the LPF 5Y is supplied to an adder 6 and to a buffer 7Y. The signal supplied to the buffer 7Y is outputted as a Y signal. In the meantime, the color-difference signals R-Y and B-Y stored in the respective frame memories 2R and 2B are read out and, then, balanced-modulated by a balanced modulator 3 which operates at a sampling frequency 4 fsc, thereby forming a digital chrominance signal. The digital chrominance signal is converted into an analog signal by a D/A converter 4a and the analog signal is applied to a band-pass filter (BPF) 5a, and a signal passed through the BPF 5a is supplied to the adder 6 and to a buffer 7b. The signal supplied to the buffer 7b is outputted as a chrominance signal. The adder 6 adds the luminance signal to the supplied chrominance signal, and the resultant signal is outputted from a buffer 7a as a composite video signal.
However, if the above-described conventional signal processing device is to be made adaptable to both the NTSC television system and PAL television system, there is the problem that since the color subcarrier frequencies fsc used in both systems greatly differ from each other, it is extremely difficult to arrange the signal processing device so that it can operate with a sampling clock signal of the same frequency for both the NTSC television system and PAL television system.
More specifically, if the aforesaid device is adapted to the NTSC television system, the horizontal resolution becomes approximately 500 lines when the sampling frequency fs for the Y signal is 4 fsc (=14.3 MHz), and the frame memory 2Y needs a capacity of 2.8 Mbits in the case of 8 bits per sample. On the other hand, if the aforesaid device is adapted to the PAL television system, the sampling frequency fs for the Y signal becomes 4 fsc (=17.7 MHz), the horizontal resolution becomes approximately 620 lines, and the frame memory 2Y needs a capacity of 3.5 Mbits in the case of 8 bits per sample. Accordingly, if the aforesaid device is adapted to the PAL television system, a memory of large capacity is needed as the frame memory 2Y when compared to the case where the device is adapted to the NTSC television system.
To solve the above-described problem, it may seem useful to adopt a method of reducing the required capacity of the frame memory 2Y for the PAL television system by digitizing the Y signal at the sampling frequency 4 fsc (=14.3 MHz) if the Y signal is to be made to conform to the NTSC television system, or at the sampling frequency 3 fsc (=13.3 MHz) if the Y signal is to be made to conform to the PAL television system. However, such a method still has a number of disadvantages. For example, if the digital balanced modulator 3 is to be operated at the sampling frequency 3 fsc (=13.3 MHz) to balanced-modulate the color-difference signals R-Y and B-Y, a circuit for forming a factor .sqroot.3/2 by calculations is needed in the digital balanced modulator 3 to multiply 4 fsc by .sqroot.3/2. As a result, a complicated circuit is needed, and to arrange the signal processing device so that it can be adapted to both of the NTSC and PAL television systems, a circuit for switching the sampling frequency between 4 fsc and 3 fsc becomes necessary.